A conventional hot-rolling process of a steel plate involves mainly billet reheating, hot-rolling and curling. During the heating process, the steel billet is heated to necessary temperature by burning natural gas in a reheating furnace. Because of the severe oxidation atmosphere of the environment, a thick oxidation scale (normally several mm thick) will be formed on the billet surface during the heating process. The oxidation scale is called as a primary oxidation scale and can be removed by a hydraulic descaler near the reheating furnace exit.
On the transfer stage during rough rolling process, more scales will be formed on the steel surface. The formed scales are removed by high-pressure water in every or some passages. Normally, it takes 7 or more passages to complete the rough rolling. Then, “an intermediate billet” having a thickness of 19-45 mm (mostly between 20 and 30 mm) will be formed after hot rolling. Then the billet is subject to finish rolling. The oxidation scale formed during and after the rough-rolling is called “a secondary oxidation scale,” and is removed by another hydraulic descaler at the entrance of a finish rolling machine. The intermediate billet is rolled into a sheet having a thickness of 1.2-12 mm (mostly between 2.0 and 4.5 mm) by the finish rolling machine. “A tertiary oxidation scale” will be formed on the steel surface during and after the finish-rolling process, and will be curled together with a strip steel and kept to room temperature. During the cooling process after curling, because of the existence of oxygen in the environment, the scale will keep on growing, thus its structure will change accordingly.
According to statistics, during the production in metallurgical enterprises, the oxidation burning loss reaches 0.5-2.5% because of high-temperature oxidation of the steel surface after each heating. In the mechanical manufacture, the loss is about 5% of the billet weight during forging. In the heating process, when furnace gas or temperature is not appropriately controlled, or the billet stays too long in high-temperature zone, especially when temperature is not adjusted in time in case of rolling failure, the thickness of scale will increase, normally to 1-5 mm or even 10 mm.
The ferric oxide scale due to high temperature oxidation directly causes loss to steel output and quality. Meanwhile, if the ferric oxide scale is not removed in time, it will be pressed into the billet surface during rolling, and cause product surface defects or even product scrap in case of severity. The formed ferric oxide scale makes precision forging impossible. In order to remove the ferric oxide scale from the steel billet surface, high-pressure water flushing, pickling and shot blasting apparatus are needed, thus production processes are added and energy consumption is increased. Besides, the ferric oxide scale will fall onto the bottom of the reheating furnace when the steel billet (ingot) is heated, and this not only corrodes the furnace and reduces its life expectancy, but also increases heavy physical work when cleaning the scale, and moreover, it will consume a lot of energy if water is used to remove dregs. During steel heating, the oxidation will not only result in ferric oxide scale, but also cause deficiency and decarbonization of alloy element in the steel at the same time. Changes in steel surface chemistry contents will deteriorate mechanical performance and corrosion resistance properties, thus lowering passing rate of the finished products.
Therefore, appropriately adopting high-temperature anti-oxidation technology, especially high-temperature anti-oxidation coating, enhances the steel yield rate in a certain extent during the production process and directly improves the yield and quality of steel. At the same time, reduction of ferric oxide scale greatly simplifies production processes, so that energy consumption in the production process will be greatly reduced, and iron ore resources will be fully utilized to improve the productivity.
The main factors influencing the high-temperature oxidation of steel include heating temperature, heating time, furnace gas composition, the chemical composition of steel and the ratio of steel heated region and steel weight, and so on. During production in metallurgical factories, adopting rapid heating, control of gas composition and inert gas protection can reduce the oxidation of steel in a certain extent, but there are some technical limitations, in large metallurgical enterprises, the corresponding equipment investments are enormous, and the operability is not ideal. The method of using protective coating is very economical and easy to be operated.
The high temperature anti-oxidation coating requires high temperature resistance, and its main component includes some metal or non-metallic oxides with good high temperature resistance properties. The common preparation method is slurry coating, firstly, the raw materials of various components for the coating are mixed to prepare slurry of good floatability, then the slurry is sprayed or brushed onto the surface of the metal matrix, and the slurry is adhered to the surface and formed a thin layer of slurry, and after being dried in shade, powder in the slurry is formed into a layer of powder coating on the surface of the metal matrix through the binder. In its original state, the coating is porous and not compact, so under atmospheric conditions and room temperature, the atmospheric components of oxidation will diffuse to the surface of the metal matrix through this porous coating and react with the metal to make the metal oxidized.
Therefore, in the original state of the slurry coating at room temperature, high temperature anti-oxidation coating can not serve to protect the metal. When the metal coated with high temperature coating is heated, with the increase of temperature, the coating is gradually dehydrated and dried, then sintered, and the coating thickness becomes thinner, pore size of the coating decreases, porosity decreases gradually and permeability decreases. When reaching the softening temperature of the coating, the coating begins to be softened and melted. The coating porosity sharply decreases and density increases. With further increase of the temperature, the coating melts to the liquid state, and then a dense airtight liquid adhesive layer is formed. Evidently, high-temperature anti-oxidation coating prepared by the slurry coating method experiences two states in the heating process, that is, in short heating and temperature rising period, the coating powder is heated and sintered, and the coating is gradually densified; and in the longer temperature rising and retaining period, the coating powder melts and is in a melting state to form a dense airtight liquid adhesive layer on the surface of the metal matrix to protect the metal matrix.
Germany currently uses coating to protect rough billet when heating forging, so 2.5% of steel can be saved every year. According to the information from the former Soviet Union hammer and sickle Dnepr special steel plant, etc., in the heating before rolling, coating is used to protect X15H5 steel billet, and the metal consumption coefficient in the plant is reduced by 25-30%, besides, the steel surface rolling quality is improved; for some easily oxidized steel, rolling leads to formation of ferric oxide scale, and 30˜70% of the steel surface has to be cleaned. In the heating before rolling, use of protection coatings can reduce the oxidation burning loss to 1/10˜ 1/30, and significantly reduces the decarbonization and improves the steel surface quality, and also greatly reduces the amount of labor of clean-up and waste rate.
Nowadays, a general steel production process transports billets after the continuous casting to the reheating furnace by track or pulls the billets into the track with an insulation cart, and then put the billets into the furnace for reheating. Before being put into the furnace, all the billets are maintained at a temperature of 800° C.-1000° C. and red hot. The existent anti-oxidation spraying process is generally suitable for spraying or brushing of the steel billet at room temperature, and the steel billet is heated in furnace after being dried at room temperature; this process normally exists in enterprises purchasing steel billets then hot-rolling them, but now the majority of steel enterprises have adopted the hot delivery technology, the existing spraying of anti-oxidation coating at room temperature is not suitable for the steel billet under the hot delivery process conditions. It is impossible for the metallurgical enterprises to cool the billet from the hot state to the room temperature before operation just for spraying anti-oxidation coating, because viewed from the aspect of energy waste, the anti-oxidation is of little significance. The existing coating spraying equipment is a simple manual wall coating spraying equipment, and it is difficult to be used in high-temperature environment. Hence, the coating spraying equipment and spraying method involved in the existing steel anti-oxidation coating does not match the actual steel rolling production process because of being limited by the film formation by spraying, brushing and drying at room temperature and the static high temperature protection, thus the promotion of the anti-oxidation spraying technology is limited.
Therefore, it is necessary to provide an anti-oxidation spraying method and spraying equipment to meet the needs of anti-oxidation spraying of steel billet under the conditions of high temperature in the steel hot rolling process.